1. Field of the Invention
The present invention relates to a method of manufacturing high-purity polycrystalline silicon granules which is suited for use in a CZ process.
2. Description of the Background
As manufacturing methods of polycrystalline silicon, the Siemens process and the fluidized bed granulation process are available. According to the Seimens process, which is a type of chemical vapor deposition (CVD) process, chlorosilane in gaseous state is fed into a reactor, while heating by energization a thin silicon rod which is arranged in a bell jar reactor. The chlorosilane gas fed into the reactor forms silicon by thermal decomposition/hydrogen reduction, which is then deposited on the thin silicon rod, thereby effecting silicon growth. This method of manufacturing polycrystalline silicon is currently a mainstay, however, the method affords a low efficiency, being basically a batch system. In addition, this method is problematic because the silicon deposition surface area is small, as compared with the reactor capacity, and heat radiation from the bell jar reactor surface is large.
At present, a fluidized bed granulation process is being developed for the manufacture of polycrystalline silicon. According to the fluidized bed granulation process, as shown in FIG. 4, deposition of silicon is made on granular silicon particles 2 as the material in a cylindrical reactor 1 which is called fluidized bed reactor. Thus, this method is, in principle, a type of CVD process, similar to the Seimens process.
According to this method, the inside of reactor 1 is heated by outside heaters 3 and into reactor 1, are fed, from above, silicon particles and, from below, the material gas containing chlorosilane. The silicon particles 2 fed into reactor 1 form a fluidized bed with the reaction gas which is rising inside reactor 1. The material gas is heated by the heaters in the process of rising inside the reactor 1, thereby undergoing thermal decomposition/hydrogen reduction, yielding silicon, which is deposited on the surface of the silicon particles 2, which form the fluidized bed.
The fluidized bed granulation method of manufacturing polycrystalline silicon is a continuous system and the ratio of the silicon deposition surface area to the capacity of the reactor is drastically larger, as compared with that in the Seimens process. A notable advantage in productivity and power consumption is also obtained. Consequently, this enables a large reduction of manufacturing cost. Since the high purity polycrystalline silicon manufactured by the fluidized bed granulation process is granular, it will, in all likelihood, have applications such as the material for solar batteries, or as the charging material in manufacturing single crystalline silicon by the CZ process. Czochralski (CZ) process is a method of producing single crystals from molten material and is used to prepare silicon single crystals.
Important parameters in the fluidized bed granulation process include fluidized bed temperature, chlorosilane concentration in the material gas, material gas flow velocity, diameter of polycrystalline silicon particles, fluidized bed height and the time taken by silicon particles to pass through the fluidized bed, for example.
With regard to the material gas temperature, over-heating at the preheating stage will cause deposition; therefore the preheating temperature should be kept below 300.degree. C.
The material gas temperature inside the reactor is controlled to about 900.degree.-1,100.degree. C. for prevention of silicon deposition on the reactor wall.
The chlorosilane concentration is controlled to about 20-50%, because at higher concentrations, fine powders are formed called fume, which brings about inter-particular cohesion.
The gas flow velocity is selected at 60-200 cm/sec, taking account of overall reaction efficiency, productivity and operational troubles.
The average diameter of fluidized particles, is preferably greater than 0.8 mm. If it is smaller than 0.5 mm, the flow rate of the gas parts where particles are dense, as specified by the minimum fluidizing velocity (Umf), so that even if the material gas feed rate is increased, it will tend to blow through as bubble gas. As a consequence, the silicon deposition velocity does not increase in proportion to the increasing amounts of the material fed. Thus, the average diameter of fluidized particles should desirably be higher than 0.8 mm, more preferably higher than 1 mm.
The dwell time or residence time of the reaction gas in the fluidized bed is empirically selected at about 0.5-2.0 sec. The dwell time is determined by dividing the height of the fluidized bed by the flow velocity of the material gas .
Under these conditions, the silicon deposition velocity is around 0.1 .mu.m/min. It is at this level that the development of the fluidized bed granulation process is presently occurring.
The silicon deposition rate means the average deposition rate, G, which is obtained by dividing the volume increase rate of silicon by the total surface area of silicon particles. The volume increase rate is obtained by dividing the weight increase rate by the density .rho.(2.33 g/cm.sup.3) of silicon crystal. The total surface area, S, may be determined, for example, from the average particle size, d, by S=V(1-.epsilon.) . (6/d) or S =(W/.rho.).multidot.(6/d). Where V designates bulk volume of particle, W weight, and .epsilon. voids.
Despite the progress being made with the fluidized bed granulation process, it has recently been determined by the present inventor that about 50-200 ppm by weight of chlorine remains in the polycrystalline silicon manufactured under such condition. The residual chlorine is not only absorbed by the surface of the granules, but exists also in their interior in abundance. On this account, the residual chlorine can not effectively be removed by after-treatments such as a vacuum high temperature treatment, for example. If chlorine remains in the interior of the granules, as the granules are charged into molten silicon in a crucible, the chlorine inside the granules will expand; thereby scattering the molten silicon. As a result, adaptation of such a product to many processes, such as the CZ process, is not feasible.
Thus, a need continues to exist for a method of manufacturing high purity polycrystalline silicon granules, which granules have a greatly reduced residual chlorine content.